1. Field of the Invention
Embodiments of the present invention generally relate to a process for forming solar cells and solar cell modules.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or multicrystalline substrates, sometimes referred to as wafers. Because the amortized cost of forming silicon-based solar cells to generate electricity is higher than the cost of generating electricity using traditional methods, there has been an effort to reduce the cost required to form solar cells.
FIG. 1A depicts a cross sectional view of a conventional crystalline silicon type solar cell substrate, or substrate 110 that may have a passivation layer 104 formed on a surface, e.g. a back surface 125, of the substrate 110. A silicon solar cell 100 is fabricated on the crystalline silicon type solar cell substrate 110 having a textured surface 112. The substrate 110 typically includes a p-type base region 121, an n-type emitter region 122, and a p-n junction region 123 disposed therebetween. The p-n junction region 123 is formed between the p-type base region 121 and the n-type emitter region 122 to form a solar cell 100. The electrical current generates when light strikes a front surface 320 of the substrate 110. The generated electrical current flows through metal front contacts 108 and metal backside contacts 106 formed on a back surface 125 of the substrate 110.
A passivation layer 104 may be disposed between the back contact 106 and the p-type base region 121 on the back surface 125 of the solar cell 100. The passivation layer 104 may be a dielectric layer providing good interface properties that reduce the recombination of the electrons and holes, drives and/or diffuses electrons and charge carriers back to the junction region 123, and enhances light absorption in the cell by reflecting back the light at 121 and 104 interface. The passivation layer 104 is drilled and/or patterned to form openings 109 (e.g., back contact through-holes) that allow regions 107 of the back contact 106 to extend through the passivation layer 104 to be in electrical contact/communication with the p-type base region 121. The regions 107 may be formed through the passivation layer 104 so that they are electrically connected to the back contact 106 to facilitate electrical flow between the back contact 106 and the p-type base region 121. Generally, the back contact 106 is formed on the passivation layer 104 by a flood printing metal paste process, pasting metal into the openings 109 formed in the passivation layer 104. The typical flood printed or blanket deposited silver (Ag) or aluminum (Al) layer, which is used to form the rear electrical back contact 106, covers most if not the entire rear surface of the substrate 121. Due to benefits gained by use of a simplified manufacturing process, which include the elimination of the need to align the flood printed material with the formed openings 109, the flood printed back contact 106 typically includes an excessive amount of the expensive flood printed paste material to perform the task of collecting and carrying the generated current from the rear surface of the solar cell.
There are various approaches for fabricating the active regions and the current carrying metal lines, or conductors, of the solar cells. Manufacturing high efficiency solar cells at low cost is the key for making solar cells more competitive for the generation of electricity for mass consumption. The efficiency of solar cells is directly related to the ability of a cell to collect charges generated from absorbed photons in the various layers. A good passivation layer can provide a desired film property that reduces recombination of the electrons or holes in the solar cells and redirects electrons and charges back into the solar cells to generate photocurrent. It can also serve the purpose to reduce the reflection if it is used for front side or transmission or if it is used on the back side of cell. When electrons and holes recombine, the recombination energy is lost as heat energy, thereby lowering the conversion efficiency of the solar cells.
Currently, most conventional solar cells use silver (Ag) to form the electrical contacts on the front and busbar/wider contacts on rear surfaces together with blanket Al metal. The silver contacts are soldered to ribbon wire, or “strings,” with conventional flux and solder materials, which is expensive and unreliable for certain types of contacts, such as fired or fire-through metal paste type contacts. Since cost is an important driver in the solar industry, it is desirable to find a way of forming a lower cost solar cell and solar cell module. One way to do this is to have fewer silver and aluminum contacts, which reduces the metal cost of the entire cell, and substitute the rear side silver contacts (called “backbus” pads) with limited area aluminum (Al) contacts. However, the aluminum backbus contacts are harder to make reliable soldered connections to, so there is a need for an innovative approach that can reliably make a stable conductive bond to the aluminum backbus contacts on the solar cell.
Therefore, there exists a need for an improved method and apparatus for manufacturing solar cell devices that have a desirable device performance as well as a low manufacturing cost.